Try online, you can do it online up to 7 days before (which means today if it's due on the 10th).
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App:
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Online doesn't necessarily work for everyone but it's worth a try. I was able to get it working (after several failures) today.
If you don't do it, like others say it should just be a 2,000B fine with no other negative consequences. I wouldn't personally go back to Bangkok for it at the moment.